1. Field of the Invention
This invention relates to the field of a network of differential optical-fiber sensors for use as vector gradiometers.
2. Description of the Prior Art
Optical-fiber sensors are used in many applications such as in measurement of the pressure field in a marine environment, due to an acoustic wavefield propagating through the water. As is well known, if an optical fiber is exposed to a pressure field, changing pressures change certain physical characteristics such as the length and index of refraction of that fiber. If a beam of continuous-wave coherent electromagnetic radiation preferably optical is launched equally into two fibers and if one of those fibers is exposed to a transient pressure field, a relative phase shift will occur between the wave forms of the two beams. The phase shift is a measure of the variations of the transient pressure field. When the two radiation beams emerging from the two fibers are combined, a variable-intensity interference pattern results. A photodetector, probing the interference pattern, will see variations of light intensity or amplitude proportional to variations in phase shift between the beams, which is, in turn, a measure of the variations in the transient pressure field.
The change in the physical characteristics of the fiber is small per unit length of fiber. Hence a large length of the fiber is wound into a compact coil. A fiber length of 50 to 100 meters per coil is not uncommon. An additional advantage of use of a coil rather than an elongated fiber is that the local pressure field effectively acts on a point receiver rather than on a distributive sensor.
Examples of typical optical-fiber differential sensors are disclosed in U.S. Pat. No. 4,162,397 to Bucaro et al, and U.S. Pat. No. 4,320,475 to Leclerk et al.
The two-legged differential sensors above discussed are useful for many applications where scalar quantities are involved. They do not lend themselves to arrays of differential sensors for measuring vector gradients in two or three dimensions. One version of a gradiometer-type sensor array is disclosed U.S. Pat. No. 4,547,869, assigned to the assignee of this invention. However, the mechanization of that patent is substantially different from the invention disclosed herein.